American astronomer John Wheeler was the first person to use the words 'Black Hole' way back in 1967. In 1971, 4 years after the term was used, the first Black Hole was discovered.
Clouded in mystery these highly concentrated and dense structure's vary greatly in size. Smaller Black Holes are formed when a star collapses into itself causing it to become very compact and dense. If you could imagine the size of the sun suddenly being compacted into the size of New York. This extreme density is so compact that even light won't escape from it. This is why they are so black and hard to see. Activity around a Black Hole caused by it's energy is the only way to know that a Black Hole is present.
Black Hole's have a very strong gravitational force and will pull in anything in it's way. Space dust and gasses from the galaxy will get sucked in to it which in turn allows the Black Hole to keep growing in size.
The larger Black Holes happen when several or even hundreds of smaller Black Holes merge together to form a super Black Hole. This can also happen when clusters of star's collapse at the same time.
Super Black Holes, which can be a million times bigger than the sun are believed to be at the very center of every galaxy, which also includes the Milky Way.
As Black Holes are almost invisible, Scientists and Astronomers find it very difficult to observe and locate them. The only way they can be noticed is by radiation that is emitted from them by the way of dust and gas.
A Black Hole is kind of like a Tornado in Space made up of compact matter. At the mouth of the Black Hole is an area known as The Event Horizon. If an object passes this area then it'll not get back out.
Sometimes depending on the angle of entry objects may ricochet of the Event Horizon and shoot at extreme speed in the opposite direction. This can cause an extremely bright jet of light and energy that can be seen from great distances.
The Black Hole has been a fascinating phenomenon for years now and it is also one that has been categorized as Science Fiction. But nowadays it is not so much about the fiction but more about the science.
There still is that fascination with what would happen if we entered a Black Hole. Maybe it's some form of Wormhole that will take us to some distance galaxy. Maybe they are there to act like doors to take us to different areas of the Universe to save us time and fuel. The truth is though that Black Holes have the capability to rip apart a passing star so unfortunately there's not much hope for us humans to have a look inside.